Can Java be used for Machine Learning?

Answer 1

Sure, most developments in machine learning run on Java or C++ underneath. At least those time-critical ones.
To name a few publications on this:
https://www.packtpub.com/big-data-and-business-intelligence/machine-learning-java
https://www.packtpub.com/networking-and-servers/neural-network-programming-java
https://www.packtpub.com/big-data-and-business-intelligence/java-deep-learning-essentials

Answer 2

yes, you can also use python

Answer 3

I use Weka. https://www.cs.waikato.ac.nz/ml/weka/